1. Technical Field
The field of the currently claimed embodiments of this invention relates to metal detection, and more particularly to detecting counterfeit or altered bullion, coins, or metal.
2. Discussion of Related Art
Coin and bullion investors and dealers need a means of quickly verifying the metal content of coins and bullion in a transactional environment. They need a device that allows for quick selection of a metal or alloy type, a straight-forward way to place the coin or bullion on the measurement device, and a fast and concise display of the result.
XRF spectrometers come closest to meeting the above-described needs. XRFs cost about $20,000, are very slow to operate, and only measure the surface of the sample to a depth of about 100 millionths of an inch. They are easily fooled by plating and cladding. XRF devices have wear-out mechanisms that result in maintenance costs. They cannot be moved to coin shows or different locations, especially in public, because they are x-ray sources and need special permits to operate, with the permit specifying the location of operation. Also, they do not work well with coins because, during the manufacture of alloy coins, some of the metals are concentrated at the surface of the coin, so the XRF reading of the elements is not in correct proportion to the actual metal contained in the bulk of the coin.
Other methods that can be used to measure the metal in coins and bullion include chemical tests and specific gravity tests. Chemical tests are time consuming, expensive, and remove material from the coin or bullion under test. The removal of material affects the value of the sample, and thus methods such as chemical tests are never used on coins and bullion. Chemical tests are also typically messy and require replacement of the chemicals, and so are expensive. Additionally they take a long time to perform. Specific gravity measurements, an alternative to chemical tests, require complex placement of the coin or bullion into a chamber that is typically filled with water. The process is very time consuming and complex. Accordingly, neither of these methods is typically used in a transactional environment because they are slow, expensive, and possibly destructive.
For very large bullion, often a hole is drilled and a bolus of material is removed. The removed metal is then chemically tested, typically using atomic absorption, mass spectrometry, atomic emission, or another well-known method. The disadvantages of this method are that it is extremely expensive, time consuming, requires metal to be removed from the bullion, and only tests a very small fraction of the bullion.
Another method for testing large bullion is ultrasound. However, ultrasound does a poor job of determining metal type, and is primarily useful for detecting large inclusions in the bar. If the bar is a fairly consistent alloy, the ultrasound system must measure the speed of sound in the metal, which may be difficult due to variations in the thickness of the bar and the roughness of its surfaces. Securing a matching fluid to couple the ultrasound waves to the bar may also be difficult. Matching liquids need to be used to make the measurements which is very inconvenient.
A detection device is needed that is fast, portable, and non-destructive.